Synthetic fixtures generated with Pillow 12.3.0, no game artwork.
PNG and lossless WebP: four literal RGBA pixels (255,0,17,0), (3,127,241,1), (63,19,211,128), (23,47,71,254).
JPEG: uniform RGB (30,80,120), quality100, no subsampling. Animated WebP adds a solid RGBA (1,2,3,255) frame to exercise explicit rejection.
Progressive JPEG and lossy WebP: same uniform RGB, quality100.
